Oligocarbonate polyols can in principle be prepared from aliphatic polyols by reacting with phosgene, bischlorocarbonic esters, diaryl carbonates, cyclic carbonates or dialkyl carbonates. Such polyols are important precursors for producing plastics, paints and adhesives. They are reacted, for example, with isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides.
DE-A 101 30 882 describes a two-stage process for preparing oligocarbonate diols, in which dimethyl carbonate (DMC) is first reacted with one or more aliphatic diols at a pressure of 1.5 to 100 bar and a temperature of 100° C. to 300° C., in the course of which the methanol formed in the reaction is removed from the reaction together with DMC as a mixture. In the second step, the terminal hydroxyl groups are decapped by applying pressures of 1 to 1000 mbar and temperatures of 160° C. to 250° C. for several hours. The preferred reaction temperature for the decapping step is 200° C. and the pressure 100 to 200 mbar. Depending on the variant, the residence time of the reaction mixture at 200° C. is between 9 and 50 hours. The thus prepared oligocarbonate diols, at a number-average molecular weight Mn of 2000 g/mol, have an OH number (OHN) of about 56 mg KOH/g. However, the actual OH functionality of the thus obtained products deviates from the theoretical value of 2.00. The reason for this is the formation of by-products having undesired end groups which lower the functionality, for example methyl ester, methyl ether, vinyl groups and others.
In many subsequent applications in which oligocarbonate polyols are used, it is not only the OHN but also the actual OH functionality (fOH) and in particular their consistency which are of particular significance. When the functionality deviates by more than 0.10 from the theoretical value (for example 2.00 for oligocarbonate diols), this leads, as a consequence of the fractions of monofunctional oligocarbonates which function as chain terminators in polymerization reactions, to materials having distinctly worsened mechanical properties. It is therefore necessary, in addition to the classical characteristic parameters such as viscosity, OHN, color number, etc., in particular to keep the actual OH functionality constant and close to the theoretical value of, for example, 2.00 for bifunctional oligocarbonate polyols.
Moreover, the transesterification catalysts described in DE-A 101 30 882 have a high activity towards compounds containing isocyanate groups in the further reaction of the oligocarbonate diols as a polyurethane raw material. This property is particularly marked when aromatic (poly)isocyanates are reacted at elevated temperature with oligocarbonate polyols which have been prepared with titanium transesterification catalysts, as is the case, for example, in the preparation of cast elastomers or thermoplastic polyurethanes (TPU). This may lead to the pot life or reaction time of the reaction mixture being shortened to such an extent that it is no longer possible to use such oligocarbonate polyols for these fields of application. In order to prevent this, the transesterification catalyst remaining in the product on completion of synthesis is very substantially deactivated in at least one additional production step. However, in particularly sensitive fields of application, even this deactivation is not sufficient to obtain adequately long pot lives or reaction times.